Device for the positive protection of a thermionic tube



P 1968 H. LEBOUTET ETAL 3,403,342

DEVICE FOR THE POSITIVE PROTECTION OF A THERMIONIC TUBE Filed Sept. 14, 1966 5 Sheets-Sheet 1 P 1968 H. LEBOUTET ETAL 3,403,342

DEVICE FOR THE POSITIVE PROTECTION OF A THERMIONIC TUBE Filed Sept. 14, 1966 5 Sheets-Sheet 2 FIG. 2

Sept. 24, 1968 H. LEBOUTET ETAL 3,

DEVICE FOR THE POSITIVE PROTECTION OF A THERMIONIC TUBE 3 Shets-Sheet 5 Filed Sept. 14, 1966 United States Patent 3,403,342 DEVICE FOR THE POSITIVE PROTECTION OF A THERMIONIC TUBE Hubert Leboutct, Saint-Cloud, and Jacques Sotfer, Plessis- Robinson, France, assignors to Commissariat a lEnergie Atomique, Paris, France Filed Sept. 14, 1966, Ser. No. 579,348 Claims priority, application (France, Sept. 23, 1965,

5 Claims. for. 328-8) ABSTRACT OF THE DISCLOSURE The present invention relates to a method for providing positive protection of a thermionic tube (valve) to which is applied a short high-energy electrical pulse which has previously been stored in a memory system, as well as to a device for the practical application of said method.

The invention is primarily directed to the protection of high-power thermionic tubes which produce a pulse-modulated highfrequency wave and is intended to be employed in modulators such as those which control the operation of certain types of particle accelerators or a radar transmitter.

In some modulators, and especially those which are known as hard tube modulators, the energy which is stored in a reservoir capacitor is very substantial and is liable in the event of accident to result in damage either to the tubes employed (klystron or switching tube) or to another component. In order to prevent such an occurrence, the energy contained in the reservoir capacitor is dissipated in a resistor having a predetermined value or so-called protection resistor, with the result that the capacitor is controlled and any accident of a serious nature is thereby prevented.

The triggering of protection devices of known type is usually initiated by a pulse which is supplied to the tube which operates at a high frequency. However, a drawback which is common to all such devices lies in the fact that, if one element thereof were to fail, the device could no longer operate and the operator would not receive advance warning of the fact.

The present invention has for its object a method of protection as well as a device for the practical application of said method which makes it possible to overcome the above-noted disadvantage.

The method in accordance with the invention consists in transmitting the energy to a thermionic tube (valve) through a pulse transformer when a trigger pulse is applied to the control grid of a switching tube, then in applying to the trigger electrode of a discharge device the two electrodes of which are connected via an impedance to the end electrodes of the memory system two pulses having the same duration and in opposite phase, the amplitude of one pulse being proportional to the current in said transformer and the amplitude of the other pulse being proportional to the current of said trigger pulse.

It has been explained that the invention also relates to a device for the practical application of the method which has been outlined above. Said protection device ice comprises a switching tube (valve), the anode of which is connected on the one hand to a high-voltage source through an inductance and on the other hand to the primary winding of a pulse transformer, the secondary winding of which is connected to the thermionic tube to which energy is to be transmitted, through a reservoir capacitor which is connected in series with a small protection resistor.

The aforesaid device is characterized in that the assembly which comprises the protection resistor and the reservoir capacitor is shunted by the series-connected or parallel-connected assembly which comprises a discharge device having means for triggering the discharge device into electrically conductive condition by ionization control, usually a trigger electrode, an impedance and the trigger electrode of said discharge device is connected to a tapping of the secondary winding of the pulse transformer via the secondary winding of an auxiliary transformer, the primary winding of which is connected to the generator which produces the signal for triggering said switching tube, the polarities of the signals which appear at the secondary winding of the auxiliary transformer and at the tapping of the secondary winding of the pulse transformer being opposite.

The discharge device is constituted by a spark-gap hav ing means such as a trigger or control electrode for triggering the discharge device into conductive condition by ionization control and conductive spheres disposed at a predetermined distance from each other, a point which performs the function of trigger electrode being disposed at right angles to the common axis of said spheres and at an equal distance from each sphere.

The spark-gap is associated with a circuit of simple design.

The said two spheres are respectively connected to the first extremity of two inductances whilst the second extremities of each inductance are respectively connected to the trigger electrode by means of two parallel-connected assemblies each consisting of a resistor and a capacitor.

Finally, emphasis should be laid on the fact that the discharge device is not necessarily constituted by a sparkgap but that, by way of alternative, either a thyratron or an ignitron could be employed.

Aside from these main arrangements, the invention is also concerned with a number of different secondary arrangements which will be mentioned hereinafter and which relate in particular to one mode of construction of a device for the practical application of the' method according to the invention, said device having been employed by the present applicant.

By means of said device, two pulses are produced and the effects of these latter are subtracted when they are applied to the trigger electrode of a discharge device, one of said pulses being a fraction of the signal applied to the high-frequency tube whilst the other pulse is derived from a separate channel in parallel with the channel which delivers the pulse to said high-frequency tube.

The two pulses referred-to are superimposed directly on the trigger electrode of the discharge device which controls the protection, with the result that, should one of the two pulses fail to appear, the protection is effective. It will be noted that this holds true whether the fault arises from the actual operation of the apparatus or whether it results from the operation of the safety system.

The circuits for generating pulses which have to be superimposed are carefully designed so that the pulse lengths are strictly equal and that said lengths do not exceed a predetermined value. A delay line can usefully be employed for this purpose.

In order that the technical characteristics of the invention may be more readily understood, there will now be described two-examples of embodiment of the device for the performance of the method which is covered by this application. It is to be understood that said embodiments are not intended to impose or imply any limitation on the practical uses for which they may be contemplated.

In theaccompanying drawings:

FIG. 1 represents an electric circuit diagram of a hard tube modulator for the application of the method according to the invention although it would be equally possible to provide a modulator which utilizes a delay line.

FIG. 2 is a block diagram of one embodiment of the invention as has been employed by the present applicant.

FIG. 3 shows the circuit which is associated with the sphere-type spark-gap which is employed in accordance with a preferred embodiment of the invention.

FIG. 4 is a detail circuit diagram which corresponds to the arrangement shown in FIG. 2.

FIG. 5 is a view illustrating the character of the pulses applied to the klystron.

FIG. 6 shows an alternate embodiment of the invention wherein a thyratron is employed as the discharge device.

FIG. 7 shows another alternate embodiment of the invention wherein an ignitron is employed as the discharge device.

The hard tube modulator A which is illustrated in FIG. 1 serves to control the operation of a klystron 2 (not shown) and is equipped with a protection device which operates according to the method of the invention. The energy which must be applied to the klystron is stored in the reservoir capacitor 4 and transmitted to the klystron via the pulse transformer 6. The energy which is employed for the purpose of charging the reservoir capacitor is supplied by a high-voltage source 8 through an inductance 10; the capacitor discharge through the transformer is initiated by a switching triode 12, there being applied to the control electrodes of said triode pulses 14 which are delivered by a pulse generator 16 (not shown) via a preamplifier 18.

It should be noted that the reservoir capacitor is connected to the primary winding of the pulse transformer through a protection resistor 20.

The two spheres of a spark-gap 22 are connected through an impedance 25 to the ends of the assembly which consists of the reservoir capacitor and protection resistor 20. The trigger electrode 24 of the spark-gap is connected to a tapping 26 of the secondary winding of the transformer 6 via the secondary winding 28 of an auxiliary transformer 30.

The trigger pulse derived from the generator 16 via the amplifier 32 is applied to the primary winding of the transformer 30. The position of the tapping 26 and the characteristics of the channel which comprises the amplifier 32 and the primary winding 34 of the transformer 30 are so determined that the two superimposed pulses 36 and 38 have the same duration.

When the trigger electrode of the triode 12 initiates the discharge of the reservoir capacitor 4 through the primary winding of the pulse transformer 6, the voltage which appears at the secondary winding of said transformer 6 is applied to the klystron 2. If said klystron is of suitable impedance and the circuits which are associated therewith are in good operative condition, there appears at the tapping 26 a pulse 36 which exactly compensates the pulse 38 which is derived from the amplifier 32 and from the transformer 30 and the potential of the trigger electrode does not vary to a sufiicient extent to trigger the spark-gap 22.

If it is assumed that, on the contrary, the klystron is not operating correctly or that the transformer 30 is not transmitting a pulse having the design characteristics, the variations in potential of the electrode 24 are sufficient to ensure that the capacitor 4 is discharged in a controlled manner by the spark-gap 22.

It will be noted that the modulator components shown in FIG. 2 which are similar to those shown both in FIG. 1 and in the other figures are designated by the same reference numerals. From a study of FIG. 2, it is apparent that the amplifier 18 is made up of two resistor stages 40-42 followed by two transformer-coupled stages 44-46, thereby making it possible by means of these assemblies to give to the incident signal the necessary level to drive the output stage 12. The end of the secondary winding of the transformer 6 which is connected to the klystron 2 is also connected to a peak-limiting system 48 controlled by a reference voltage source 50.

Reference will now be made to FIG. 3, which illustrates the spark-gap and its associated circuit in accordance with a preferred embodiment of the invention.

The spark-gap 22 is essentially composed of two spheres 52-54 which, for the sake of simplicity of the description, will be assumed to be respectively coupled to the potential +V of a voltage source and to ground through inductances 56-58. Two identical series-connected resistors 66-68 shunt the complete assembly of inductances and th spark-gap.

When voltage is applied to the two spheres of the spark-gap, equipotential surfaces 60-62-64 are established between the two spheres. The equipotential surface which corresponds to the potential V/ 2 is a plane at right angles to the common axis of the two spheres which is located at an equal distance between these latter. A point 24 which is disposed in this plane and brought to the potential V/2 by connection to the common point of the potential divider 66-67 does not perturb the electric field which is maintained between the two spheres. If a negative pulse is applied to the point 24 by means of the capacitor 70, breakdown takes place with the upper sphere 52. Said point is then brought to the potential +V and the point effect is again produced in the case of the sphere 54. In practice, it is merely necessary to apply to the point a pulse whose amplitude is in the vicinity of V/2 in order to trigger the spark-gap when the intersphere voltage varies from V to V/4. The trigger electrode 24 does not play any part inasmuch as the triggering is completed and the current flows only between the two spheres. The inductances 56 and 58 of a few microhenries must be employed for the purpose of reducing the triggering energy.

Finally, in order to establish good equilibrium of the bridge formed by the arm which comprises the sparkgap and voltage divider and in order to prevent the useful pulse from triggering the spark-gap, capacitors 72- 74 will be added in parallel across each resistor of the voltage divider.

FIG. 4 shows a hard tube modulator which is fitted with the protection device in accordance with the invention and which has been constructed by the present applicant.

It will be observed that the power stage 12 is essentially made up of two triodes -82 which are associated in parallel, this arrangement having been adopted on account of the high level of energy to be supplied to the klystron 2 at each pulse.

Instead of a single reservoir capacitor, there is employed in this example a capacitor bank consisting of five capacitors in parallel, a protection resistor being connected in series with each capacitor. For the sake of clarity, only three of these capacitors have been shown in FIG. 4.

The reservoir-capacitor bank is directly coupled to the pulse transformer 6, the secondary winding of which is constituted by two windings 84-86, one winding being employed for the purpose of supplying the necessary energy to the klystron whilst the other winding forms part of the protection circuit.

One of the major requirements which had to be met by the designers of the apparatus shown in FIG. 4 was to give a shape which was as correct as possible to the pulses applied to the klystron and, in particular, to obtain pulses having a fiat peak to within 2.l

It was found necessary to tolerate a peak drop of 5% produced by the capacitor bank and the transformer 6. Th different elements of the stages of the preamplifier make it possible to ensure that the pulse transmitted by the transformer 88 which provides a coupling between the preamplifier and the power stage has a very stable amplitude and is perfectly flat. The coupling circuit between the preamplifier 18 and the power stage 12 which comprises the resistor 90, the capacitor 92 and the resistor'94 makes it possible to produce a pulse of trapezoidal shape which compensates the defect previously referred to. Finally, the pulse-amplitude stability is improved by making use of a negative feedback amplifier 96 which couples one tapping of the secondary winding 84 of the transformer 6 to the high-voltage end of the secondary winding of the transformer 88. As can be seen in FIG. 5, in actual practice, the pulses applied to the klys'tron are not absolutely flat and the level P of the pulse does not rigorously correspond to a constant amplitude. The value limit of the variation of the relative amplitude represented by the ratio of the peak drop a to the pulse height A should be within 210- The amplifier 32 which serves to supply the primary winding of the transformer 30 which forms part of the safety circuit consists of four stages.

There are again shown in this diagram the elements of the safety circuit which are shown in FIG. 1 and which essentially comprise the secondary winding of the transformer 6 and the secondary winding of the transformer 30.

The discharge device can consist of a thyratron or an ignitron.

FIG. 6 shows a typical circuit arrangement using a thyratron having its control grid 24' connected to the secondary winding 28. The thyratron circuit is connected in shunt with capacitor 4 and the resistor through transformer 102. The anode of the thyratron is connected to one end of primary 104 of transformer 102 through series resistance 106, while the cathode of the thyratron is connected to the other end of primary 104 through resistor 108. DC operating voltage to the cathode and anode are provided from a conventional source. Secondary winding 110 of transformer 102 is connected in series with resistor 125 to complete the shunt circuit across capacitor 4 and the resistor 20.

FIG. 7 is analogous to FIG. 6 except that an ignitron has been substituted for the thyratron. Elements of FIG. 6 corresponding to elements of FIG. 5 are represented by a prime notation. For example, transformer 102 of FIG. 5 is represented in FIG. 6 as 102'.

What we claim is:

1. A device for the positive protection of a thermionic tube to which is applied a short high energy pulse previously stored in a memory system, comprising a high voltage source, an inductor and a switching tube, the inductor coupling the high voltage source to the anode of the tube, a reservoir capacitor and a protection resistor electrically connected in series, said series connected reservoir capacitor and protection resistor being shunted by a discharge device comprising means for triggering said discharge device into electrically conductive condition by ionization control, a pulse transformer, .the anode of said switching tube being coupled to the primary winding of the pulse transformer through said series connected capacitor and resistor, the secondary winding of said pulse transformer being connected to the thermionic tube to which an electrical pulse is to be applied, a trigger pulse generator adapted to be energized to supplytrigger pulses to the control grid of the switching tube; an auxiliary transformer having a primary and a secondary winding, said secondary winding being connected to one end to a tapping of the secondary winding of the pulse transformer and at the other end to said means for triggering said discharge device, an amplifier connected for transmitting trigger pulses to the primary winding of the auxiliary transformer such that said discharge device is triggered in response to the voltage across the secondary winding of the auxiliary transformer.

2. A device as described in claim 1 wherein said discharge device is a spark gap and said means for trigger ing into electrically conductive condition is a triggering electrode, said spark-gap having conductive spheres disposed at a predetermined distance from each other and said trigger electrode being disposed at right angles to the common axis of said spheres and at an equal distance from each of said spheres.

3. A device as described in claim 2, said spheres being connected directly to the first extremities of two inductances, two resistors each respectively connected at one end to the second extremities of said inductances and at the other end to said trigger electrode and two capacitors connected in shunt across said resistors.

4. A device as described in claim 1 wherein said discharge device is a thyratron and said means for triggering said discharge device into electrically conductive condition is the control electrode of said thyratron.

5. A device as described in claim 1 wherein said discharge device is an ignitron and said means for triggering said discharge device into electrically conductive condition is the control electrode of said ignitron.

References Cited UNITED STATES PATENTS 3,260,895 7/1966 Buff et al 317--16 3,275,891 9/1966 Swanson 317-16 3,277,342 10/1966 Ross 328-67 JOHN W. HUCKERT, Primary Examiner.

J. D. CRAIG, Assistant Examiner. 

